Being a warrior tank in World of Warcraft is a curious job. A few of them rise to the lofty heights of being main tank for raids. The main tank in a raid is probably the most important job in the whole raid group. Nearly all main tanks are officers in their guild, many of them raid leaders or even guild leaders. If you are a leader of men, this the most challenging, but also most interesting and rewarding job there is. Everyone in your guild knows who the main tank is, but who knows who are the best healers or damage dealers? Unfortunately there aren't many of these main tank jobs around, not everyone who is a good tank also would make a good main tank and raid leader. And if you look for other occupations for a tank, things are looking more grim.
In raids and small groups the main problem is that the number of tanks has an upper limit. You need 1 tank for a 5-man group, and X tanks for this or that raid group. Anything more is too much. If you gather a group of any size together, fill all the necessary positions, and then still have some free spots, another tank is the last thing you'd invite. What should an extra tank in a group do? As crowd control he is less efficient than a mage, hunter, or warlock. And as damage dealer he is just plain bad. If you have a 5-man group with 2 tanks, replacing the second tank with a mage for example would always be an improvement.
For soloing a warrior tank only rivals a non-retribution paladin in inefficiency. You basically attack a mob and wait that it dies of old age. Being a tank has some advantages when exploring new areas, and when being surprised by several mobs, because survivability is obviously good. But for things like farming, daily quests, or most normal quests, being a tank just means you do everything much slower than everyone else.
PvP roles for a warrior tank are even more limited. It would be hard to kill you in melee, but even with spell reflection and shield bash you end up being killed by spells most of the time. And your ability to harm other players is very limited. Abilities like intimidating shout or hamstring are useful, but they aren't special to protection spec warriors. So apart from defending a flag in Arathi Basin, a tank isn't really useful in PvP, and even there a paladin would probably be better.
So now imagine the average guy who rolled a warrior, played him to the level cap, and is playing as a tank. He'll probably find that the job of main tank in his guild is taken, and there is already quite some competition from paladins and druids for the offtank role, so its hard to get a spot in a raid. He'll be welcome in 5-man groups, but only as long as they don't have a tank yet, and nowadays there aren't quite as many 5-man groups going as last year, because most people got everything they needed from dungeons already. He is soloing badly. And in PvP he barely performs better than the guy who is AFK in the entrance cave, and mostly plays the unfun role of dummy target or being ignored. Most people will react to that situation by either playing another class, or by at least doing a respec to a talent build that is more useful. Arms for PvP, Fury for soloing, or some hybrid for both. Who would want to play a spec which is only good for waiting for a 5-man group?
What we end up with is a tank shortage. Yes, there are main tanks, but they are wearing a nice set of raid epics, and aren't interesting in tanking in a 5-man dungeon. They have to prepare the next raid after all. And all the warriors you ask are now Mortal Striking in PvP instead of tanking.
And that is a general situation on most servers, not just a statistical fluke. Player behavior is influenced by game design, so if game design steers players away from being a tank, there is a tank shortage everywhere. Protection spec warriors have simply been too narrowly designed for their tanking role in a group. And the more World of Warcraft is moving away from group play and into pseudo-solo PvP play, the less attractive the protection spec becomes. And that is a vicious cycle, because the less tanks there are, the harder it gets for everyone else to find a group, and the more attractive soloing becomes. Funnily enough Blizzard could promote grouping by making the protection talent tree of warriors more useful in soloing and PvP. Or by allowing people to have two different specs between which it is easy and free to switch. But if the game continues as it is, the tank shortage will become even more pronounced in the future. You have been warned!